1. Field of the Invention
The present invention relates generally to infrared thermographic systems and specifically to improved long-range viewing through such systems.
2. Discussion of the Prior Art
Imaging systems are used to visualize at least a portion of the landscape by means of infrared radiation in the frequency spectrum at a wavelength longer than one micron, which spectrum includes visible light. In the visible light range, known television cameras utilize electron beam scanning in a conventional manner. Visualization in the infrared range can be by day or night but is generally more useful at night when direct vision in the visible range is reduced or impossible. The spectral bands of the radiation detected by these thermographic systems correspond to atmospheric transparent windows, at wavelengths of either 3 to 5.mu. or 8 to 12.mu.. The latter spectral band is particularly suited to thermographic systems because it is perfectly adapted to imaging bodies at ambient temperature, because a black body at 300.degree. K. has its maximum emission at about 10.mu.. Systems that employ optical/mechanical raster and line scanners are generally known, as discussed in La thermographie infrarouge [Infrared Thermography] by G. Gaussorgues Part 1, published by Technique et Documentation 11 rue Lavoisier, 75386 Paris Cedex 08, France on Dec. 2, 1980, herein incoporated by reference.
The sensitivity of present infrared cameras is inadequate for certain operations, such as sky, land or sea horizon surveillance, especially when atmospheric conditions are poor, or when long-range detection (on the order of several dozens of kilometers) is desired.
The conventional procedure for improving the sensitivity of infrared cameras is to sum successive images. It will be noted that the human eye itself performs this processing, accumulating at the most four to five successive images. If i is the number of images accumulated, the gain in the signal/noise ratio is .sqroot.1. The eye thus provides a sensitivity gain on the order of 2. On the other hand, if the accumulation is made electrically by summing and averaging 16 successive images, for example, the eye no longer can be regarded as an integrator, for the image now is updated too slowly. So, the apparent gain for an observer, with respect to a normal functioning of .sqroot./2, is 2 even when there is an accumulation of 16 images.
It will be noted that this image accumulation processing can be performed at the digital level by an electronic processor without in any way changing the functioning of the camera's raster and line scanners with respect to the normal functioning of the system. The major inconvenience of the above accumulation procedure is the need for a stable sight. However, even assuming that a stable sight is provided, a moving target in the field of view can become blurred, depending upon the amount and direction of movement. The duration of 16 images would be 0.64 second at 25 images (50 rasters) per second, which corresponds to the European television standard. Image stability could be improved tracking a reference, selected by the operator, or the target itself after detecting by the concatenation method and measurement after this detection. The blurring effect due to motion thus will be less, but at the expense of the recognition range because of the concatenation processing and the consequently long image updating time due to accumulation processing by images.